Glazed solar collectors

ABSTRACT

Solar collector apparatus is described in which solar radiation is collected in a glazed cavity which may also include a transpired solar collector layer. Air warmed in the cavity may be used for space heating within buildings or diverted to heat management systems which facilitate, for example, heat storage. The glazing, particularly coated glazings, improve the performance of the devices by allowing solar energy to enter the cavity and preventing heat loss, and by negating the effect of ambient wind on the transpired solar collector layer.

The invention concerns apparatus and method for collecting solar radiation and converting same to a form of energy suitable for space heating in buildings.

The use of passive solar energy to heat buildings is well known. In September 1881, E S Morse obtained U.S. Pat. No. 246,626 for “Warming and Ventilation Apartments by the Sun's Rays” and the general concept of warming air beneath a glazing, before directing the warmed air to the interior of the building to provide space heating, has been developed during the intervening years. So called “Trombe Walls” typically comprise a glazing which is arranged in spaced apart relation with a building wall to define a heated cavity. Air enters the cavity via one or more vents near its bottom and is warmed by solar radiation which causes it to rise and exit the cavity via a vent to the interior of the building.

WO 2010/086126 discloses another approach to building heating using solar energy. In this document the solar collector comprises a collector panel which defines an at least partially enclosed space. The collector panel, known as a transpired solar collector, comprises energy absorbing sides, air-inlet openings connecting the outside environment with the at least partly enclosed space and an air-outlet duct connected to the at least partly enclosed space. Solar energy is absorbed by the energy absorbing sides and this causes heating of air in the outside environment, in the immediate vicinity of the panel.

A negative pressure is applied to the air-outlet duct and this causes the warmed air to pass through the air-inlet openings, through the at least partially enclosed space and out of the air-outlet duct, from where it is directed to the point of use in space heating, or heat storage. WO 2010/086126 also describes systems for storing and managing the heat derived from the panels.

The effectiveness of the apparatus described in WO 2010/086126 is affected by wind which disturbs the warmed air in the vicinity of the panel, before it passes through the inlet openings.

Chinese Utility Model No. eN 201517854 U describes another system in which air, warmed by solar radiation, is used for space heating. Again, a panel is disclosed, with holes through which warmed air passes and is recovered for building heating. The apparatus also includes a glazing, arranged in parallel spaced relationship with the panel so that air between the glazing and the panel is warmed by solar radiation before being passing through the holes. A glazing comprising glass having a high iron content is recommended because such glass absorbs solar radiation more effectively, better to heat the air between the glazing and the panel.

The requirement for ever improving means for collecting solar energy remains. The present invention provides such means, whereby solar energy is collected with a high degree of efficiency and made available for uses such as internal space heating in buildings or heat storage and management e.g., according to the methods described in WO2010/086126.

According to the invention, apparatus for collecting solar energy comprises:

a cavity, a glazing, arranged to allow solar radiation to pass therethrough and enter the cavity, an inlet aperture, arranged to allow air to enter the cavity from an external environment and an outlet aperture, arranged to allow air warmed by the solar radiation, to exit the cavity; characterised in that the glazing comprises a coated glass.

The coating may comprises a low emissivity coating, a pyrolytically deposited fluorine doped tin oxide a titania coating or an antireflection coating.

In embodiments comprising a single glazing sheet, the coating is located on either the internal surface, or the external surface of the glass, relative to the cavity.

In one embodiment, the glazing comprises a major sheet and has integral glass sidewalls. Two glazings may be employed, each glazing comprising a major sheet and two integral glass sidewalls, and the glazings being arranged to define the cavity therebetween.

The apparatus may include an Insulated Glazing Unit (IGU), said IGU comprising at least two glass sheets. The IGU may comprise two glass sheets wherein the coating is located on surface #4 of the IGU. The coating on surface #4 of the IGU may comprise titania or pyrolytically deposited fluorine doped tin oxide.

Alternatively, the IGU may comprise two glass sheets wherein the coating is a low E coating located on surface #3 of the IGU. This coating may be the product of a sputter deposition process and may comprise silver or a compound of silver.

Alternatively, the coating may be located on surface #1 of the IGU and may comprise an antireflection coating, a titania coating or a pyrolytically deposited fluorine doped tin oxide.

The IGU may include a glass sheet comprising a tinted glass. Typically, the tinted glass would have a composition comprising at least 0.15 wt % iron.

The apparatus may include at least one sheet of glass comprises a low iron content glass, having an iron content of less than 0.02 wt %, preferably less than 0.015 wt % most preferably less than 0.01 wt %.

The apparatus may include a transpired solar collector arranged to divide the cavity into first and second regions, the first region being bounded by the glazing wherein the collector comprises a sheet of material having a plurality of holes, the holes providing communication between the two regions, and wherein the inlet aperture is arranged to allow air to enter the first region and the outlet aperture is arranged to allow air to leave the second region.

Where the apparatus includes a titania coating on the innermost glass surface relative to the cavity, means may be included for directing a flow of water on to said surface.

According to a second aspect of the invention, apparatus for collecting solar energy comprises:

a cavity, a glazing, arranged to allow solar radiation to pass therethrough and enter the cavity, an inlet aperture, arranged to allow air to enter the cavity from an external environment and an outlet aperture, arranged to allow air warmed by the solar radiation, to exit the cavity; characterised in that the glazing comprises a low iron glass.

Preferably, the glass comprises less than 0.02 wt % iron, more preferably less than 0.015 wt % iron and even more preferably, less than 0.01 wt % iron.

The invention will now be further described using non-limiting examples, with reference to the attached figures in which:

FIGS. 1-3 illustrate various embodiments of the invention employing a single glazing on a simple undivided cavity;

FIGS. 4-8 illustrate various embodiments of the invention employing a multiple glazing on a simple undivided cavity;

FIGS. 9-14 illustrate various embodiments of the invention in which the cavity is divided by a transpired solar collector;

FIG. 15 illustrates apparatus according to the invention in which a glazing having a ‘U’ shaped cross-sectional profile is employed;

FIG. 16 illustrates apparatus employing a glazing having a ‘U’ shaped cross-sectional profile wherein the cavity is divided by a transpired solar collector and

FIGS. 17a and 17b illustrate embodiments wherein two glazings having a ‘U’ shaped cross-sectional profile are used to define the cavity.

FIGS. 18 and 19 illustrate a test apparatus used to evaluate the performance of glazed solar collectors according to the invention both with and without a Transpired Solar Collector sheet;

FIG. 20 shows the performance of various embodiments of the invention in terms of rise in air temperature achieved for a given solar radiation and

FIG. 21 shows the improvement in performance over a simple uncoated glazed cavity that may be achieved by various embodiments of the invention.

Referring to FIG. 1, a first embodiment of the invention includes a cavity 1 which is partly defined by a glazing 2. The cavity further comprises side walls (not shown) parallel to the plane of the page and may be arranged against the wall of a building which serves as another wall of the cavity. Alternatively, an insulated panel 3, which may be fixed to the exterior of a building, may serve as the back wall of the cavity. Examples of insulated panels which might serve this purpose include Trisomet® 333 manufactured by Tata Steel.

During operation, cold air enters the cavity via a lower aperture 4, is warmed by solar radiation passing through the glazing and then exits cavity via an upper aperture 5. After exiting the cavity, the warmed air may be directed to the interior of a building to provide space heating or the heat may be extracted for storage or other use by means well known to persons skilled in the art.

Warming of the air in the cavity 1 is aided by the so-called greenhouse effect, whereby solar radiation passes through the glazing and is absorbed by the interior of the cavity to cause heating. The glazing is less transmissive to infrared radiation associated with this heating and hence inhibits heat loss. A low iron glass such as Pilkington Optiwhite™ may be used for the glazing as this further reduces the amount of solar energy absorbed by the glass and facilitates transmission of energy to the cavity. Preferably, the low iron glass comprises <0.02 wt % iron, more preferably <0.015 wt % and most preferably <0.010 wt %.

In various embodiments of the invention, a coating 6 is applied to the inner surface of the glazing.

In one embodiment, the coating 6 comprises a low emissivity (low E) coating (designated 6 a in subsequent drawings) which reduces the transmission of infrared radiation. Thus loss of heat through the glazing, from within the cavity, is reduced.

The low E coating is preferably chosen to optimise the trade-off between solar gain and heat trapping by balancing transmission of solar radiation into the cavity against heat loss by infrared radiation from the cavity. In this connection, an optimum sheet resistance of between 15 and 30 ohm per square has been established. Pilkington K Glass™ is a low E glazing material suitable for use in this embodiment of the invention. K Glass™ comprises a fluorine doped tin oxide coating on a colour suppressing silicon oxycarbide coating.

In another embodiment the coating 6 may comprise a functional coating (designated 6 b in subsequent drawings) such as titanium dioxide (titania). Titania coatings are known to offer an antimicrobial effect via a photocatalytic effect, whereby highly reactive free radicals are generated from oxygen and moisture under the influence of ultraviolet radiation in (for example) sunlight. Inclusion of such a coating on the inner surface of the glazing causes decomposition of organic contaminants and killing of microorganisms to provide a purified air supply for the building.

Use of such a coating along with means for irrigating the inner surface of the glazing provides for easy cleaning of the inner glazing surface. Such irrigation means (not shown) typically comprises a water outlet such as a nozzle, array of nozzles or a pipe with one or more holes, connected to a water supply and arranged to direct a flow of water on to the coated surface.

The self-cleaning action is enhanced where a hydrophilic coating is used which causes water droplets to spread on the surface and run off easily.

Pilkington Activ™ is a titania-based self-cleaning glazing which serves in this embodiment.

In general, in all of the embodiments described herein, where a titania-based self-cleaning coating is used on a interior surface of the glass, relative to the cavity, such embodiments may optionally include means for irrigating said inner surface.

Referring to FIG. 2, in another preferred embodiment, a functional coating 6 b is applied to the external surface of the glazing. For example, an antireflective coating increases the effectiveness of the device by increasing the amount of solar radiation which is transmitted through the glazing 2 to provide heading of air in the cavity 1. Pilkington Optiview™ serves as a suitable glazing product in this embodiment.

Alternatively, the functional coating 6 b could comprise a self-cleaning coating such as found on Pilkington Activ™ so that the exterior glass surface remains relatively free from contaminants that would otherwise inhibit transmission of solar radiation and adversely affect the visible appearance of the installation. Pilkington Activ™ also offers anticondensation properties in this orientation as its hydrophilic properties cause water droplets which gather on the outer surface to spread and readily run off.

Anticondensation properties are also achieved using pyrolytically deposited fluorine doped tin oxide based coatings such as found in Pilkington K Glass™ or Energy Advantage™. These so-called ‘hard coatings’, which are deposited during the float glass manufacturing process, are highly stable and fused to the glass surface. This makes them suitable for applications where they are exposed to the external environment.

Referring to FIG. 3, the glazing could include coatings on both sides. The inner surface coating 6 could, for example be a low E coating or a titania coating. The coating 6 b on the external surface could be a functional coating such as anticondensation, antireflective or self-cleaning. Preferably in this embodiment, the emissivity of the inner coating 6 is less than that of the outer coating 6 b.

Referring to FIG. 4, the invention may include an insulated glazed unit (IGU) 7 of the type well known in the art and comprising two or more glazings 2, 8 held apart by spacers 9 to define a low thermal conductivity gap 10, typically containing air or another gas. In the embodiment shown, a coating 6 is shown on the innermost glass surface which, as in the previous embodiment (FIG. 1) might be a low E coating or a titania coating. By convention, the glass surfaces are numbered from #1 to #4, number 1 being the outermost (i.e. furthest from the building, contacting the external environment) so in FIG. 3, the coating 6 is shown on surface #4. When used in this specification, conventional numbers referring to the various glass surfaces are underlined in order to distinguish them from numerals used to identify features in the drawings.

Referring to FIG. 5, as an alternative to the arrangement shown in FIG. 3, a low E coating 6 a may be located on an internal glass surface with the IGU.

In FIG. 5, the low E coating 6 a is shown on surface 3. Since this surface is protected from the elements due to its location within the IGU, a so called ‘soft coating’ may be used such as a silver-based sputtered coating of the type known in the art. Pilkington Optitherm™ is an example of a product having such a coating.

Referring to FIGS. 6 and 7, use of a multi-glazed IGU offers a number of options for the application of various coatings to the various surfaces. In FIG. 6, a functional titania coating 6 b is shown on surface #4 along with a low E coating 6 a on surface #. Thus, the benefits of both reduced heat loss and air purification are realised in a single embodiment.

In FIG. 7, a low E coating 6 a is shown on surface 3 along with a functional coating 6 b on surface #1. The functional coating 6 b could inter alia be a titania coating; an antireflection coating or an anti-condensation coating.

Referring to FIG. 8, the invention may employ a sheet of tinted glass 2 a as the inner sheet 1 of the IGU 7. Tinted glass may be realised as a glass having a higher iron content. Tinted glasses are well known to those skilled in the art and typically contain more than 0.15 wt % Fe. The ratio of Fe²⁺/Fe³⁺ varies and other additives may also be varied. The tinted glass 2 a absorbs more energy from the solar radiation causing it to become heated. This heat may be transferred to the air in the cavity 1 by re-radiation or conduction. The benefits of using a tinted glass are enhanced when used in conjunction with a low E coating 6 a, particularly on surface #3, which inhibits re-radiation of the absorbed energy away from the cavity 1.

Apparatus according to the invention may also include a transpired solar collector comprising a perforated sheet, arranged to absorb energy from solar radiation and cause warming of the air in the vicinity of its surface.

Referring to FIG. 9, in one embodiment of the invention, the cavity is divided into two regions 1 a and 1 b by a transpired solar collector panel 11. The collector panel 11 is arranged to absorb energy from solar radiation causing its temperature to rise. This, in turn gives rise to heating of the air in the immediate vicinity of the collector.

A negative pressure is applied to region 1 b of the cavity, via outlet aperture 5, by means not shown (for example an extractor fan) and this causes warmed air to pass through the perforations (holes) in collector 11 from region 1 a to region 1 b. The warmed air is then directed as described previously for the functions of space heating or heat storage.

Inclusion of the glazing 2 enhances the effectiveness of the apparatus by the greenhouse effect as previously described. As before, a low iron glass such as Pilkington Optiwhite™ is preferred as this increases the effectiveness of the glazing in this regard. In addition, the glazing prevents dispersal of the warmed air by wind so that it is able to pass through the transpired solar collector panel 11 undisturbed.

The single glazed system illustrated in FIG. 9 preferably includes a coating 6 on the glazing. In one embodiment a low-E coating is used, further to reduce heat loss by radiation from the cavity region 1 b to the external environment. In another embodiment, the coating 6 comprises a titania coating to provide purified air as described previously.

The transpired solar collector may also be used in conjunction with an IGU. As before, the IGU offers a number of options in terms of surfaces to which various coatings might be applied.

FIG. 10 illustrates embodiments of the invention in which a transpired solar collector panel 11 is combined with an IGU. Surface #4 of the IGU bears a coating 6, which in one embodiment is a low E coating. In another embodiment, coating 6 comprises a titania coating.

In the embodiment illustrated by FIG. 11, a low E coating 6 a is applied to surface #3 of the IGU.

In FIG. 12, an embodiment is shown which employs a transpired solar collector panel 11; a low E coating 6 a is applied to surface #3 of the IGU 7 to reduce heat loss and a titania coating 6 b is applied to surface #4 in order to provide purified air.

In FIG. 13, the IGU 7 has a low E coating on surface #3 and a functional coating 6 b on surface #1. Functional coating 6 b could be an antireflective coating, a titania coating or an anticondensation coating.

Referring to FIG. 14, the invention may employ a transpired solar collector panel 11 and an IGU 7 in which one of the glass sheets 2 a is tinted. As before, the tinted glass has a greater ability to absorb solar radiation which may then be transferred to the air in cavity as heat and a low E coating is preferably applied to surface #3 of the IGU 7, to reduce radiation of heat away from the cavity.

Referring to FIG. 15, in another variant of the invention, a glazing is employed which has a ‘U’ shaped profile, that is it comprises a major sheet 2 a (which corresponds to the flat glazing 2 shown in previous embodiments) along with integral glass sidewalls 2 b. Such a glazing, along with the wall of the building or insulating panel 3 defines the cavity within which air is warmed by solar radiation.

Utilisation of a glazing having integral glass sidewalls allows for more solar energy to enter the cavity. Such a glazing may be provided as Pilkington Profilit™ linear channel glass.

Referring to FIG. 16, the ‘U’ profiled glazing may be used in conjunction with a transpired solar collector panel 11 which, as in previous embodiments, divides the cavity into regions 1 a, where the air is warmed, and 1 b, from where the warmed air is drawn after passing through the transpired solar collector.

Referring to FIGS. 17a and 17b , the cavity 1 may be defined by two glazing sheets having a ‘U’ profile. In use, the major sheet 2 a of one glazing sheet faces the external environment and allows solar radiation to enter the cavity while the major sheet 3 a of the other glazing provides the back wall of the cavity.

Glazing systems employing Pilkington Profilit™ in this manner are commercially available.

As in other embodiments, various surfaces of the glazings may be coated. For example, inner surface of major sheet 3 a may include a low e coating. Also, use of a low iron glass will facilitate greater transmission of solar radiation to the cavity. Pilkington Profilit™ is commercially available in a low e glass (Pilkington Optiwhite™).

EXAMPLE 1

Referring to FIG. 18, apparatus used to test the effectiveness of various embodiments of the invention comprised: a glazed cavity assembly as generally illustrated in FIGS. 1-8, further provided with air inlet and outlet ducts 12 and 13 respectively, a fan 14 located in the outlet duct and arranged to draw air through the cavity 1 and a thermocouple 15 arranged to measure the temperature of the air after it passed through the cavity 1.

FIG. 19 shows a similar arrangement to that in FIG. 18, except that a glazed cavity with transpired solar collector as generally illustrated in FIGS. 9-14 was being tested.

During testing, a plurality of assemblies as illustrated in FIGS. 18 and 19, and each with a different surface coating 6, were arranged side by side with the glazing facing substantially in the direction of sunlight.

N.B. In FIGS. 18 and 19, the coated surface 6 is shown on the ‘interior’ of the cavity but these figures are examples only and in some cases arrangements were used where the coated surface was on the ‘exterior’.

The air flow was maintained at a linear velocity of about 1 m/s which, for the apparatus used, equates to a volumetric flow of about 3.3×10⁻³ m³/s.

A comparison of the air temperature, as measured by thermocouple 15, with the ambient (inlet) air temperature provides a measure of the effectiveness of the glazed cavity for warming air.

Table 1 shows the coating stacks associated with each of the coatings used (all thicknesses in nm). The coated stacks were tested on a simple glazed cavity (FIG. 18) and data were also obtained using an unglazed transpired solar collector and a glazed solar collector having uncoated soda-lime silica glass (i.e. as illustrated in FIG. 19 but with no coating 6).

TABLE 1 Description of various coated products tested on glazed solar collector. Product SiCO SiO2 SnO2 SiO2 SnO2:F K Glass on 70 0 0 0 350 Pilkington Optiwhite ™ TEC6 0 20 ± 5 25 ± 3 25 ± 3 660-720 TEC10 0 0 25 ± 3 25 ± 3 380-420 TEC15 0 0 25 ± 3 25 ± 3 340 Anticondensation 0 15  17 ± 3 30 ± 3 210

FIG. 20 shows the air temperature rise obtained for each of the solar collector arrangements indicated. The data show that even the combination of an uncoated glazing with the transpired solar collector produces a heating effect that is significantly greater than that offered by either of these features alone. The uncoated glazing offers some benefit by collecting and storing solar energy by a straightforward ‘greenhouse’ effect but also protects the Transpired Solar Collector from the effects of ambient wind. As previously noted, warmed air, in the vicinity of the Transpired Solar Collector (in cavity 1 a of, e.g. FIG. 9), may be dispersed by the wind before passing through the Transpired collector, when a glazing is not present.

However, a much greater benefit is obtained by use of a coated glazing.

The Pilkington K Glass™ on Optiwhite™ produced the most significant increase in air temperature. Without wishing to be bound by theory, it is believed that these coatings showing best performance offer the optimum compromise between the need to allow solar radiation to pass therethrough and enter the cavity, and the need to retain the heat in the cavity once it has been captured. (N.B. TEC 15 R refers to the TEC 15 product being used in ‘reverse orientation’ i.e. the coated surface was on the outside of the cavity).

EXAMPLE 2

Referring to FIG. 21, a further experiment was performed to compare the performance of a coated single glazing; a double glazed unit having a coating on surface #3 and an unglazed Transpired Solar Collector, with that of an uncoated glazed solar collector, having no transpired collector. The upper plot shows the performance of the double glazed simple cavity (FIG. 18 with double glazed unit); the middle plot shows that of the single glazed simple cavity (FIG. 18) and the lower plot shows the performance of the unglazed Transpired Solar Collector.

The horizontal axis in FIG. 21 represents temperature gain achieved by an uncoated glazed solar collector having no transpired solar collector panel (i.e. a glazed cavity). The vertical axis represents the temperature gain achieved by each of the configurations described at the same point in time as the corresponding value on the horizontal axis was measured. Thus, the greater the slope of the graph, the better the performance of the configuration in question.

The coating used on both the single and double glazed cavities was an anticondensation coating produced by Pilkington Group Limited, whose composition is also indicated in table 1.

FIG. 21 shows that all of these configurations provide superior performance to an uncoated glazed collector (by virtue of the positive slopes of all plots); a single coated glazed cavity provides superior result to an unglazed Transpired Solar Collector and a further increase, of about 6%, when the cavity is double glazed.

EXAMPLE 3

Table 2 below summarises a further set of experiments performed using various embodiments of the invention.

Average Test Airflow Temp Average Number Ducting Increase Irradience Efficiency Glazing Series 1 1 0.3 23.7 850 0.19 K Glass OW #3 DGU 2 0.57 23.7 850 0.37 K Glass OW #3 DGU 3 0.62 27.7 850 0.47 22 ohm OW #3 DGU 4 0.7 18.9 850 0.36 22 ohm OW #3 DGU 5 1.03 15.4 850 0.43 K Glass S OW #3 DGU (glazed without TSC) 6 9.6 850 0.00 7 1.09 19.2 850 0.57 22 ohm OW #4 DGU (glazed with TSC) 8 1.14 11.4 850 0.35 22 ohm OW #4 DGU (wide gap) Series 2 1 0.3 15.2 621 0.17 K Glass OW #3 DGU 2 0.57 14.8 621 0.31 K Glass OW #3 DGU 3 0.62 5.4 621 0.36 22 ohm OW #3 DGU 4 0.7 12.9 621 0.34 22 ohm OW #3 DGU 5 1.03 9.9 621 0.38 K Glass S OW #3 DGU (glazed without TSC) 6 8.7 621 0.00 7 1.09 14.2 621 0.58 22 ohm OW #4 DGU (glazed with TSC) 8 1.14 9.5 621 0.40 22 ohm OW #4 DGU (wide gap) (Additional Data: Ducting diameter 0.075 0.075 vol flow at 1 m/s 0.004415625 0.004415625 density of air 1.2 1.2 mass of air flow 0.00529875 0.00529875 Power required to raise temp by 1 C. 5.3305425 5.3305425 specific heat capacity of air 1.006 1.006 area of solar collector panel 0.23 0.23) #3, #4 etc = surface 3, surface 4 etc; DGU = double glazed unit; OW = Optiwhite ™

Table 2 further illustrates the effect of the invention in providing heated air for space heat heating and heat storage.

Numbers 5 and 7 of each series illustrate the improvement that can be achieved by a combination of a coated glazing and a transpired solar collector panel, as compared with a coated glazing only. In both series, the coated glazed Transpired Solar Collector (number 7) achieved greater temperature increases and efficiencies that the corresponding glazing without transpired solar collector panel (number 7).

In this regard it should be noted that the K Glass S OW #3 DGU and the 22 ohm OW #4 DGU have been shown to be similar in performance, so the differences between numbers 5 and 7 are attributed to the presence or absence of the transpired solar collector panel. (OW 22 ohm is an anticondensation coating on Pilkington Optiwhite™). 

1-29. (canceled)
 30. An apparatus for collecting solar energy comprising: a cavity, a glazing, arranged to allow solar radiation to pass therethrough and enter the cavity, an inlet aperture, arranged to allow air to enter the cavity from an external environment and an outlet aperture, arranged to allow air warmed by the solar radiation, to exit the cavity; wherein the glazing comprises a coated glass.
 31. The apparatus according to claim 30, wherein the coating comprises a low emissivity coating.
 32. The apparatus according to claim 31, wherein the coating comprises pyrolytically deposited fluorine doped tin oxide.
 33. The apparatus according to claim 30, wherein the coating comprises a titania coating.
 34. The apparatus according to claim 32, wherein the coating is located on the internal surface of the glass, relative to the cavity.
 35. The apparatus according to claim 32, wherein the coating is located on the external surface of the glass, relative to the cavity.
 36. The apparatus according to claim 30, wherein the coating comprises an antireflection coating.
 37. The apparatus according to claim 30, comprising an Insulated Glazing Unit (IGU), said IGU comprising at least two glass sheets.
 38. The apparatus according to claim 37, wherein the IGU comprises two glass sheets and the coating is located on surface 4 of the IGU.
 39. The apparatus according to claim 38, wherein the coating comprises titania.
 40. The apparatus according to claim 38, wherein the coating comprises pyrolytically deposited fluorine doped tin oxide.
 41. The apparatus according to claim 37, wherein the IGU comprises two glass sheets and the coating is a low E coating located on surface 3 of the IGU.
 42. The apparatus according to claim 41, wherein the coating is the product of a sputter deposition process.
 43. The apparatus according to claim 42, wherein the coating comprises silver or a compound of silver.
 44. The apparatus according to claim 37, wherein the coating is located on surface 1 of the IGU.
 45. The apparatus according to claim 44, wherein the coating comprises an antireflection coating.
 46. The apparatus according to claim 44, wherein the coating comprises a titania coating.
 47. The apparatus according to claim 44, wherein the coating comprises pyrolytically deposited fluorine doped tin oxide.
 48. The apparatus according to claim 37, wherein at least one glass sheet comprises a tinted glass.
 49. The apparatus according to claim 48, wherein the tinted glass has a composition comprising at least 0.15 wt % iron.
 50. The apparatus according to claim 30, wherein at least one sheet of glass comprises a low iron content glass, having an iron content of less than 0.02 wt %.
 51. The apparatus according to 30, wherein the glazing comprises a major sheet and has integral glass sidewalls.
 52. The apparatus according to claim 30, wherein the cavity is divided into first and second regions by a transpired solar collector, the first region being bounded by the glazing, said collector comprising a sheet of material having a plurality of holes, the holes providing communication between the two regions, and wherein the inlet aperture is arranged to allow air to enter the first region and the outlet aperture is arranged to allow air to leave the second region.
 53. The apparatus according to claim 33, further comprising means for directing a flow of water on to the titania coated glass surface.
 54. The apparatus according to claim 51, comprising two glazings, each glazing comprising a major sheet and two integral glass sidewalls, the glazings arranged to define the cavity therebetween.
 55. An apparatus for collecting solar energy comprising: a cavity, a glazing, arranged to allow solar radiation to pass therethrough and enter the cavity, an inlet aperture, arranged to allow air to enter the cavity from an external environment and an outlet aperture, arranged to allow air warmed by the solar radiation, to exit the cavity; wherein the glazing comprises a low iron glass.
 56. The apparatus according to claim 55, comprising glass comprises less than 0.02 wt % iron.
 57. The apparatus according to claim 56, comprising glass comprises less than 0.015 wt % iron.
 58. The apparatus according to claim 57, comprising glass comprises less than 0.01 wt % iron. 